A key research focus of my laboratory is to elucidate the role of PAMPs in the biology of the pathogen. We have shown that the rice XA21 PRR detects Ax21, a PAMP produced by Xoo. Ax21 is a sulfated, Type I-secreted protein (Lee et al., Science). Ten Xoo rax genes are required for activation of Xa21-mediated immunity (Burdman et al., 2004; Goes da Silva et al., 2004; Lee et al., 2006, Lee et al., 2008). Collectively, our data suggest that Ax21 serves as a peptide quorum-sensing signal that plays a key role in virulence. To date, such peptide signaling has only been demonstrated in Gram-positive bacteria so this discovery is quite significant. Furthermore, tyrosine sulfation has not previously been demonstrated in any prokaryotic species. In the next few years, we will further elucidate the role of Ax21 in bacterial cell-cell signaling and virulence gene expression and test if Ax21 binds bacterial/and or host receptors.